Accelerated plasma source



Sept. 1, 1959 w. R. BAKER ACCELERATED PLASMA SOURCE Filed Sept. 6, 1956 SOURCE TRIGGER GENERATOR 48 TRIGGER SWITCH DC POWER SUPPLY D.C. POWER SUPPLY TRIGGER SWITCH IN VEN TOR.

HIGH

0 VOLTA GE 0 PULSE GENERATOR WILLIAM R. BAKER BY ZM4JM ATTORNEY.

United States Patent ACCELERATED PLASMA SOURCE William R. Baker, Orinda, Calif., assignor to the United States of America as represented by the United States Atomic Energy Commission Application September 6, 1956, Serial No. 608,411

8 Claims. (Cl. 313-61) The present invention relates to method and means for generating a plasma and for accelerating the plasma.

It will be appreciated that the problem of accelerating a plasma, recently of importance in the field of nuclear physics, poses difiiculties in that a plasma constitutes a space charge neutralized ionized gas including both electrons and positively charged ions so that any attempt to influence a plasma by electric fields produces plasma disintegration. Thus, for example, electric field of such polarity that it would attract positively charged ions in a plasma would repel electrons so that the field would terminate the plasma instead of accelerating same. Attempts to accelerate plasmas otherwise than above noted as by magnetic fields have proven unsatisfactory for a variety of reasons.

The utilization of a plasma at the site of generation as is commonly done suffers the disadvantage that neutral particles in the form of un-ionized gas molecules contaminate the plasma. This condition is particularly disadvantageous for many plasma applications and is overcome by the present invention which accelerates the plasma by means having no effect upon neutral particles and at a rate in excess of gas difiusion so as to remove the plasma from such contamination. The present invention operates to accelerate a plasma by means of a self-generated magnetic field. There is herein employed an effect sometimes termed a pinch efiect" which is utilized to apply an accelerating force to a plasma.

As regards the nature of the pinch efiect herein employed, analogy may be made to the simple example of a pair of parallel conductors carrying current in the same direction. It will be appreciated that each of said conductors generates a magnetic field circulating thereabout by virtue of current flow therethrough, and a summation of the fields of the two conductors shows that the fields tend to cancel between the conductors so that the resultant field is stronger outward of each conductor. Elemental theory shows that this stronger magnetic field outward of the conductors creates a force or magnetic pressure on the conductors tending to force same together. The conductors are thus pinched together.

The above-noted phenomena may be observed in electric discharges through space or gas. Considering an arc .discharge wherein positive ions move in one direction and electrons in the other direction, there will be seen to be formed the analogy of a plurality of currentcarrying conductors wherein current fiow is in the same direction so that a pinch effect results.

It has been found in the present invention that for large discharges and, in particular, for intense plasmas the pinch elfect is appreciable. The unbalanced selfgenerated magnetic fields of an intense plasma pinches "the plasma to compress same. The present invention operates to harness the force of the pinch effect not only for compressing a plasma but also for accelerating same in a predetermined direction whereby the plasma moves F ce as a unit from a generating area outward to a utilization point.

Advantages of the pinch effect in operating upon a plasma are evident in that the oppositely charged particles of the plasma are similarly effected thereby so that there results a composite plasma effect without plasma termination or disintegration. As to the magnetic field generation, it will be appreciated that negatively charged electrons moving in one direction produce magnetic fields in the same direction as do positively charged ions moving in the opposite direction, so the resultant field is analogous to that produced by a bundle of conductors carrying current in the same direction and, likewise, the resultant field acts upon the positive and negative charges similarly owing to their opposite directions of motion.

It is an object of the present invention to accelerate a plasma by self-generated magnetic fields.

It is another object of the present invention to generate a plasma in such a manner that compressive forces of self-generated magnetic fields also urge the plasma along a line of travel.

It is a further object of the present invention to generate a plasma having opposed edges inclined with respect to each other for accelerating the plasma along a line by self-generated magnetic fields.

It is yet another object of the present invention to provide means for accelerating a plasma away from contaminating neutral particles.

Numerous other advantages and possible objects of the invention will become apparent to those skilled in the art from the following description taken together with the accompanying drawing, wherein:

Figure 1 is a sectional view of a preferred embodiment of the tube of the present invention with associated circuitry; and

Figure 2 is a partial view in section of the tube and showing an alternative gas source.

Considering now the structural details of the preferred embodiment of the invention illustrated in Fig. 1 of the drawing, the improved pinch tube 11 thereof, as it is hereinafter termed, includes an electrically conducting base member 12 having a stepped periphery mating with a cylindrical insulator 13 extending above the base. The insulator 13 is secured to the base as by bolts 14 extending upward through the base into the cylinder about the lower end thereof and sealed thereto as by a seal 16. A second cylinder or electrode 17 formed of metal and having a lesser diameter than the insulating cylinder 13 is disposed coaxially therewith above same and extending a ways into the top of the insulating cylinder to form an annular passage 18 therebetween.

Mounting of the electrode 17 is accomplished from the insulating cylinder 13 by means of a manifold 19 comprising an annular member secured atop the insulator 13 by bolts 21 through the manifold into the insulator end and defining with the electrode 17 an annular gas chamber 22. The manifold 19 may be formed of metal and is vacuum sealed to the insulator as by a seal 23 therebetween and joined to the electrode 17 as by welding thereto. The manifold is provided with a radial gas inlet passage 24 connecting with an exterior gas source 26 as by a conduit 27 and gas egress from the manifold is accomplished via a plurality of spaced apertures 28 in the bottom of the manifold communicating between the chamber 22 thereof and the annular gas passage 18.

Evacuation of the pinch tube may be accomplished from the top of the electrode 17 as by means of an annular vacuum manifold 29 secured thereabout and communicating therewith through radial apertures 31 spaced about the electrode. Vacuum pumping means 35 is connected by sealed piping to the vacuum manifold for drawing a vacuum thereon and thence evacuating the pinch tube through the radial apertures 31 about the top of the electrode, the size and disposition of these apertures being optional. The electrode 17 is open ended and the pinch tube is adapted for connection to plasma utilization apparatus at the upper end thereof. The vacuum manifold 29 may thus serve further as an attachment flange, being fixed to the electrode 17 as by welding and having transverse bolt holes 32 therein to retain fasteners, not shown.

The lower portion of the pinch tube may be encompassed by a housing 33 formed of metal as a cylinder extending below the base plate 12 and defining a chamber 34 thereat. The housing is secured to the abovedescribed tube assembly by means of an inwardly extending flange about the housing top, which flange extends between the annular gas manifold-insulator juncture as in a manifold recess and is therein retained by the bolts 21 passing therethrough. Within the housing chamber 34 below the base plate 12 there is disposed a spark gap switch 36 including upper electrode comprising a hemispherically curved boss 37 on the base plate under surface and a like electrode 38 spaced beneath same. A trigger electrode 39 is disposed intermediate the two spark gap electrodes 37 and 38 for controllably actuating discharge therebetween.

As regards the energizing and control circuitry of the pinch tube 11 there is provided a direct current power supply 41 having one terminal grounded and the other terminal connected through a resistor 42 to the lower spark gap electrode 38 in insulated relation to the tube housing 33. The spark gap switch is energized from a bank of capacitors 43 connected in parallel between the lower spark gap electrode 38 and the housing 33 and these capacitors are charged by the direct current power supply 41. Capacitor discharge through the switch is controlled by the third spark gap electrode 39 disposed intermediate the main spark gap electrodes. A voltage applied to the trigger electrode 39 of the switch 36 will cause breakdown of the gap from same to one of the main electrodes following which the entire voltage will then exist across the shorter distance from the other main electrode to the trigger electrode so as to fire across same and a spark is thereby formed between the two main electrodes. A control circuit for the trigger electrode may include a direct current power supply 46 connected through a trigger switch 47 across the primary winding of a pulse transformer 48. A trigger signal generator 49 connects to the trigger switch to actuate same and the pulse transformer secondary winding is coupled as by a capacitor 51 to the spark gap switch trigger electrode 39. In the circuit illustrated the pinch tube housing 33 is grounded, as is one end of both primary and secondary transformer windings and one terminal of the control power supply 46 so that a control signal is applied between the spark gap trigger electrode 39 and lower electrode 38. As regards polarity, it is intended in the illustrated embodiment that the base 12 shall comprise a cathode so that it is made negative with the positive terminal of power supply 41 grounded; however, alternative polarity connections are possible.

Operation of the above-described pinch tube is initiated by the trigger generator 49 which applies a signal to the trigger switch 47, that may comprise a thyratron, and causes switch 47 to connect the power supply 46 across the primary winding of the transformer 48. The trigger signal from the generator 49 need only be momentary in duration as the power supply voltage is thus momentarily applied through the transformer 48 between the spark gap trigger electrode 39 and high voltage electrode 38. The high voltage electrode 38 is maintained at a high voltage by the capacitors 43 previously a pulse voltage to the trigger electrode 39 of the opposite polarity increases the potential between the electrodes 38 and 39 far beyond that required for breakdown whereby a heavy spark fires between same and thence to the upper electrode 37. As the base 12 is electrically joined to the spark gap electrode 37 the former is energized by spark gap conduction to impress a very large voltage between the base and the electrode 17 which is electrically grounded by connection to the housing 33.

Within the volume defined by the insulator 13 and base 12 and comprising an arc chamber 52, there is provided a gas to be ionized. This gas is derived from the source 26 and flows via the manifold 19 through the bottom openings 28 therein down about the outside of electrode 17 through the annular passage 18 into the arc chamber 52. The gas pressure in the chamber 52 is preferably maintained at about 100 to 1000 microns of mercury by continuous evacuation via the pump 35. The large spark gap voltage impressed between the base 12 and electrode 17 produces an electron discharge therebetween which immediately becomes an ionizing are by passage through the gaseous atmosphere and there is thus defined between the base 12 and electrode 17 a high intensity plasma.

Immediately upon arc discharge initiation and plasma formation the pinch effect above described operates upon the plasma. With the base 12 negative, ions flow toward the base 12 and electrons toward the electrode 11 to form the equivalent of a plurality of conductors carrying current in the same direction so that magnetic pressure results with a net force directed radially inward. This strong self-induced magnetic field about the plasma compresses the lower end thereof but does not materially compress the upper end for the upper discharge terminus is ring shaped. Consequently, the plasma attains the general configuration of an inverted cone with the small end adjacent the base. As the plasma intensity is much greater at the cone vertex so also is the self-induced magnetic field strength greater about the small diameter plasma and thus the magnetic pressure operates to produce a net upward force upon the plasma as well as a radially compressive force.

Another manner of viewing this phenomena is to note that charged particles travel in the conical plasma is upward and outward from the small discharge end on the base 12 and as the self-induced magnetic field exerts a force normal to current fiow the resultant force has a component parallel to the base inward of the plasma and another component normal to the base and away from same.

Whatever the theoretical approach, it remains that a net upward force is applied to the plasma which operates to rapidly accelerate same upward through the electrode 17. This phenomenon is readily verified and I have viewed through windows in the pinch tube a plasma blob very rapidly traverse the electrode 17 and emerge from the end thereof and further have made measurements beyond the electrode 17 which positively identify plasma thereat. It is further to be noted that the velocity of plasma egress is much greater than the diffusion velocity of neutral particles so that un-ionized gas in the arc chamber 52 or even in the electrode 17 is left behind by the accelerated plasma, for the magnetic fields acting on the plasma do not affect neutral particles such as gas molecules, and the plasma emerging from the electrode 17 is thus uncontaminated and contains only electrons and ions.

It will be appreciated that the above-described pinch tube is pulse operated with pulse energy being determined by the charge on the capacitors 43 and pulse interval being limited by the capacitor charging rate through the resistor 42. Inasmuch as the tube operates in a pulsed manner it may also be advantageous to employ a pulsed gas source and such a gas source is shown in charged by the power supply 41 and the application of Fig. 2. In this embodiment all tube elements are the same as those described above except for the gas supply and herein there is provided in the manifold main passage 22 a ring 61 formed of a metal having a gas to be ionized occluded or otherwise retained therein. This ring 61 extends about the circumference of the electrode 17 out of contact therewith and mounted on radially extending insulators 62 to the manifold. The ring 61 is not complete but has a short gap defined by the two ends 63. and 64. One end 63 of the ring may be grounded to the electrode 17, as schematically shown in Fig. 2,

and the other end is electrically joined to a conductor 66 that extends through the manifold via a lead-through insulator 67 to an external terminal 68. The ring 61 is heated by passing a current therethrough to drive off gas occluded therein and appropriate electrical circuitry may include a high voltage pulse generator 69 connected between the exterior ring terminal 68 and ground. This generator 69 produces spaced pulsed signals which in addition to heating the ring may also be employed to initiate actuation of the spark gap switch 36. Thus in this embodiment the trigger switch receives a signal from the pulse generator 69 rather than from a trigger generator 49. A time delay means such as a delay line 71 is connected from the pulse generator output to the trigger switch control terminal, such as a thyratron grid, so that the trigger switch is energized a predetermined time after the ring isheated to drive off gas therefrom. In this manner, suflicient time is allowed for the gas to diffuse through the manifold apertures 28 down into the arc chamber 52 before the trigger switch actuates the spark gap to produce a discharge in the pinch tube. By the aforesaid means a precisely controllable pulsed supply of gas is provided which supplies gas for ionization only when needed so that wasteful gas flow is eliminated.

What is claimed is:

1. A plasma source comprising a pair of spaced electrodes with one thereof having an aperture therein and the other being solid, power supply means establishing a discharge between said electrodes, and a gas source supplying gas to said discharge whereby same becomes an arc discharge including a high intensity plasma, said plasma generating a self-constricting magnetic field that reduces the plasma size adjacent said solid electrode whereby the same magnetic field accelerates the plasma away from said solid electrode.

2. A plasma source comprising a plate electrode and an open-ended tubular electrode spaced from said plate electrode and disposed end-on thereto, means impressing a large electrical potential between said electrodes for establishing an intense discharge therebetween, and means supplying gas between said electrodes for establishing by said discharge an intense plasma that generates a selfconstricting magnetic field accelerating the plasma through said tubular electrode.

3. A plasma source comprising a first electrode having a flat face, a cylindrical insulator extending from said fiat electrode face and defining an arc chamber, a tubular electrode directed toward the flat face of said first electrode and extending within said insulator in spaced relation to said first electrode, gas supply means directing gas into said are chamber from about the end of said tubular electrode, and power supply means impressing pulsed electrical energy between said electrodes for establishing successive arc discharges therebetween.

4. A plasma source as defined in claim 3 further defined by said gas supply means producing pulses of gas directed into said are chamber in synchronism with the pulsed energization of said electrodes.

5. A plasma source comprising an open ended tubular electrode, a second electrode disposed in proximity with said tubular electrode out of contact therewith, means supplying a gas at low pressure between said electrodes, and means impressing high voltage high current electrical energy between said electrodes for establishing an intense arc discharge having a strong encircling magnetic field constricting same on said second electrode whereby a portion of said discharge is pinched off of said second electrode and is self-propelled through said tubular electrode.

6. A plasma source comprising a metal plate, a cylindrical insulator disposed with said plate closing one end thereof, a tubular electrode extending into the open end of said insulator in spaced relation to said plate, a gas manifold about said tubular electrode supplying gas to the interior of said insulator from the exterior of said tubular electrode, and evacuation means about the outer end of said tubular electrode for evacuating the interior of said insulator and tubular electrode.

7. A plasma source as defined in claim 6 further characterized by including a spark gap switch impressing pulsed high voltage and current electrical energization between said plate and tubular electrode for establishing a plasma that is self-propelled out of said tubular electrode from said plate.

8. A plasma source as defined in claim 6 further defined by a housing about said insulator and plate and defining a chamber, a spark gap switch in said chamber and including at least a portion of said plate as a switch electrode, and means for controllably passing large pulses of electrical energy through said switch for establishing an arc discharge from said plate within said insulator.

References Cited in the file of this patent UNITED STATES PATENTS 2,271,666 Smith Feb. 3, 1942 2,285,622 Slepian June 9, 1942 2,769,096 Frex Oct. 30, 1956 2,785,311 Lawrence Mar. 12, 1957 

